Selective amplifiers having band-pass characteristic for use in the antenna input in the high frequency stages of a radio receiver include such one as shown in FIG. 1 consisting of a band-pass filter BPF and an amplifier AMP connected in cascade. In this case, the filter BPF is generally disposed before the amplifier AMP so as not to be influenced by a high level input of radiowaves out of the receiving band. While a filter BPF having, in combination, a serial resonance circuit composed of a capacitor C.sub.A and a coil L.sub.A and a parallel resonance circuit composed of a capacitor C.sub.B and a coil L.sub.B as shown in FIG. 2 is generally used, a great ripple appears in a frequency/transmission characteristic as shown in FIG. 3 if the aimed passing frequency range is relatively broader with respect to its center frequency, that is, if the specific band width is great, and the filter circuit is not preferred as such. Specifically, in the example shown in FIG. 3, since the magnitude of transmission is remarkably increased at both of the shoulders of the characteristic curve and it goes as high as to 20-30 dB in the extreme case, the uniform gain characteristic can not be obtained within the band.
In view of the above, a load resistor R.sub.L is connected between the filter output terminal P and the ground, i.e., in parallel with the filter elements C.sub.B and L.sub.B in the parallel resonance circuit as shown in FIG. 4 so that the transmission characteristic may be unified in the band as shown in FIG. 5. In such a structure, however, if the specific band width is near 1, for example, where the central frequency is 1000 KHz and the pass band is set as .+-.500 KHz relative thereto, i.e., from 500 KHz to 1500 KHz, the value of the resistor R.sub.L is set to a considerably small value to significantly decrease the gain and, as a result, thermal noises due to the resistor R.sub.L is relatively increased to remarkably lower the S/N ratio in the circuit.
It is now examined how the S/N ratio varies depending on the resistor R.sub.L in the circuit shown in FIG. 4 assuming that the coils L.sub.A and L.sub.B and capacitors C.sub.A and C.sub.B produce no losses and the amplifier AMP produces no noises. Assuming the series impedance of the capacitor C.sub.A and the coils L.sub.A as Z.sub.A, the parallel impedance of the capacitor C.sub.B and the coil L.sub.B as Z.sub.B and the thermal noise voltage of the resistor R.sub.L as V.sub.N in the circuit shown in FIG. 4, the equivalent circuit up to the point P in the FIG. 4 can be represented as in FIG. 6. Here, E.sub.S represents a signal voltage applied to the input terminal IN and E.sub.O is the out put voltage at the point P in FIG. 4. The thermal noise voltage V.sub.N is represented as: V.sub.N =.sqroot.4kTBR.sub.L in which k is Boltzmann's constant, T is absolute temperature (.degree.K.) and B is band width.
Since the signal voltage E.sub.S is divided by the resultant impedance of the impedance Z.sub.B and the resistor R.sub.L and the impedance Z.sub.A, the signal voltage E.sub.OS generated at the point P is represented as: ##EQU1## While on the other hand, since the noise voltage E.sub.ON produced at the point P is represented as: ##EQU2## the S/N ratio is expressed as: ##EQU3## While the S/N ratio throughout the entire selective amplifier is also influenced by the noises generated from the amplifier AMP connected behind the filter BPF, it can be seen that the S/N ratio depends on the value of R.sub.L in the structure shown in FIG. 4 excepting the case of Z.sub.A =0 in the equation (3) and that the S/N ratio is decreased more as the value of the resistor R.sub.L is selected smaller. Since a smaller resistor R.sub.L serves to decrease the ripple referred to above more effectively and it is selected to a much smaller value for the specific band, for example, near 1, this has a defect of worsening the S/N ratio although rendering the gain within the band approximately constant in the circuit shown in FIG. 4. Since the impedance Z.sub.A is represented as: ##EQU4## the equation (3) is expressed as: ##EQU5##
As apparent from the equation (5), since the S/N ratio is optimum only at a particular frequency of 1-.omega..sup.2 L.sub.A C.sub.A =0 in the selective amplifier shown in FIG. 4 and it decrease at Z.sub.A .noteq.0, the S/N characteristic is satisfactory only at a particular frequency as shown by the curve C.sub.1 in FIG. 10 and then worsens as it deviates therefrom. Since the S/N characteristic is in proportion to the square root of the load resistor R.sub.L, it worsens in proportion to the square root of the resistor R.sub.L as the latter becomes smaller.
While the parallel connection of the load resistor R.sub.L to the parallel resonance circuit can decrease the ripple appearing in the band-pass characteristic of the filter BPF to unify the gain in the pass band of frequency, it also provides a negative effect of lowering the filter output level. Although a particularly high S/N ratio is desired where such selective amplifier is used in the amplifying stage at the top of a receiver, that is, nearest to the antenna, it is inevitable for the S/N reduction depending on the decrease in the gain of the filter BPF due to the load register R.sub.L. FIG. 7 shows an equivalent circuit for a circuit where a field effect transistor is employed as the amplifier AMP regarding the noises therein. In this figure, v.sub.n and i.sub.n show a voltage component and a current component of the noises respectively and FET shows an ideal field effect transistor with no noises. While the field effect transistor actually has not infinite but a finite input impedance and the current component i.sub.n represents the noises generated from an equivalent resistor substituting the input impedance (constant current source). The voltage compnent v.sub.n represents the noises generated in the transistor such as SCHOTTKEY noises. As the voltage applied to the point P, that is, the output from the filter BPF decreases, the voltage component v.sub.n is increased relatively to worsen the S/N ratio.